But this is more than just a storm in a test tube and will have real-world impact across industry – most notably for the print sector with regard to the use of mercury vapour curing lamps. 

The full implications for curing wide-format print still need further clarification, but it is worth pointing out that manufacturers of the printers and their associated inks have already been investigating and adopting alternative curing technologies, and have enjoyed considerable success.  

When UV-curable inks were introduced into the wide-format digital printing sector, drying was always going to be a key requirement. Since mercury arc technology provided a workable, long-proven solution it was the obvious option.

As a technology however, it is not without its flaws. Not least the inconsistent longevity of the lamp systems and their decreasing effectiveness towards the end of their 1,000-hour lifetimes. 

Mercury vapour lamps also generate intense heat and infra-red energy that can compromise the structure and surface of substrates; even with filters and cooling devices, the temperatures can pose a problem not only to the material being cured but also to any other materials in the immediate vicinity. In addition, even when not in use, they are relatively costly to run as they require a warm-up period; in stand-by mode they still use significant amounts of electrical power.

Mercury falling

The gradual phasing out of mercury in other appliances has been gaining momentum in recent years so it was, perhaps, inevitable that alternative curing technologies were going to be developed for use in print environments. Part of this has been driven by revised legislation relating to health and safety parameters, but consideration for environmental principles has also been factored in to the reasoning for a move to new lamp technologies.

The implementation of a major change in current legislation, that not only affects Europe but is expected to be applied globally, has now been stated in the Restriction of the Use of Certain Hazardous Substances (RoHS) 2 recast directive. This differs from the REACH regulations which are widely read and have to be observed by manufacturers and suppliers that deal with toxic materials and their waste. The implications dictated by the RoHS definition have already caused concern through the wording that relates to the use of mercury arc lamps. This can be interpreted through the detail which states that its use is to be phased out on or before July 2016. But, as is common with much of the documentation that accompanies this type of notification, the exclusion list is long and loosely defined and is likely to be a source of confusion for anyone who reads it and tries to understand whether or not there will be a direct effect on present-day wide-format (or, indeed, any) UV-curable inkjet device that incorporates this kind of drying technology.

There is an alternative, of course, in the form of LED UV-curing systems, which, after a slightly faltering start in the digital inkjet market, have moved forward to represent a real alternative to the use of mercury lamps, as they now offer a host of benefits. These include a cooler curing environment, the elimination of emissions, a reduction in the instances of eye damage through exposure to UV-B and UV-C wavelengths and a significant reduction in power required to drive the curing units. Furthermore, LED lamps don’t require a warm-up time so don’t need to be left in a stand-by mode.

The potential red-flag warning suggested by news of the RoHS 2 recast should be considered, regardless of whether or not wide-format ink-jet devices are accommodated on the considered exclusion list or not. None of this is news to the manufacturers of UV-curable printers, regardless of sector, and there have been other reasons for adopting LED curing as an alternative to mercury for quite some time. 

Mike Battersby, marketing manager, large-format at Fujifilm Specialty Systems is aware of future directives. “The printing industry responds positively to changing legislation as a matter of course in terms of inks, substrates and lamp technology,” he states. “There is legislation covering substances in equipment in the RoHS directive. Mercury is restricted, but currently permitted in UV curing lamps under an exemption, although this exemption is under review.”

However, changing a machine’s curing capabilities from mercury arc to LED lamps isn’t simply a case of swapping out old for new. Photoinitiators are intended to react with the lamp’s spectral peaks to polymerise and form a hard, dry result on the material’s surface. This is the result of the reaction to the relevant UV-B and UV-C wavelengths via inks that are intended to work with the mercury vapour curing. With LEDs, the reliance lies on UV-A which has a relatively narrow wavelength within the overall spectrum and the ink chemistry must echo this in its formulation in order to cure satisfactorily on the
media surface. 

“The benefits of mercury arc lamps are higher intensity UV output and currently lower cost. LED light emitters require a tight tolerance in distance from the media,” continues Battersby. “Additionally, there is greater ink formulation freedom with mercury arc lamps as more raw materials are available for this well-established technology. As well as multiple suppliers in the market, the lamps offer a range of wavelengths giving greater choice depending on ink selection and application needs.”

Ergo, the key to successful LED curing doesn’t simply lie in the lamps but in the inks that need extra ingredients in order to work effectively. In simple terms, this involves the inclusion of amines or aminoacrylates to absorb the oxygen generated by LEDs or by nitrogen that provides an inert environment that generates a cure within the required UV-A spectral range. This complicates the factors involved with successful results across different media.

EFI’s Ken Hanulec, vice-president, marketing, Inkjet Solutions, concurs: “Manufacturers need to consider form, fit and function when considering LED curing, including the targeted application set, and if this technology will adequately solve proper ink curing, intended materials and overall performance of a given solution.”

Hanulec factors in additional considerations. “Mercury arc and LED lamp systems emit different light waves; therefore, photoinitiators (the catalyst for curing) in the ink formulations differ. With that said, at EFI we develop inkjet printing systems and inks in unison to ensure superior colour management, image quality and durability, and consistent system performance and reliability, regardless of curing technology. We select and grind our own pigments for the brightest colours and custom-formulate each ink set to perform as intended on a broad variety of wide-format, label and narrow web printing applications.”

Scientific jargon aside, the fact lies that if mercury arc lamps are going to disappear from the market in the foreseeable future, then manufacturers and ink producers must factor in the change to usable formulations which present a greater challenge, per se, than merely swapping out one set of lamps for those of another type. If the sea change is imminent, users of wide-format, and other, print devices can rest assured that the industry hasn’t been caught napping and that developments in ink chemistry have been moving towards LEDs for some years. 

Growing popularity

The transition should be relatively straightforward. Indeed, the successful use of LED curing has been apparent for some years and its popularity is growing. Early criticisms and compromises have largely been overcome and the concentration by some manufacturers in the relatively early stages, including Roland DG, Mimaki and EFI, is now being echoed by those machine developers who formerly felt that this technology could not supersede satisfactorily its mercury arc equivalents.

Combinations of mercury arc and LED curing and the effect on accuracy overall are also acknowledged by EFI. “Colour management, and all the functions around colour and profiling are more a result of the ink set and capabilities of the digital front-end or RIP,” Hanulec comments. “The printhead technology and printing system software drive accurate drop placement, while dot gain is controlled through different levels of curing. On the Vutek HS100 Pro, for example, we control dot gain using LED lamps (known as pinning) to achieve variable gloss levels. The final cure is achieved at the end of the print process using mercury arc lamps.”  

From the supplier viewpoint, where distributors are reliant on available technologies to pass onto their customers, even though there are questions hanging over the future of UV curing, this type of printer technology remains the fastest growing market segment. 

Robin East, director of CMYUK, has long involvement with both mercury arc and LED devices. “We are seeing an exponential shift of interest to LED curing from customers. But much of this is because they want to print to a more diverse range of materials, including thinner substrates and heat-sensitive substrates,” he says. “Greener production methods are to be encouraged, of course, as are cost savings with power usage being an obvious area in the desire to lower energy use. Most users aren’t aware of the possibility of legislative changes; they rely on their suppliers to be up-to-date with directives and to sell compliant equipment that falls within the scope of the law.”

UV-curable technology is not going to go away and is a staple production methodology across many print disciplines. Regardless of legislation or lamp type, the growth in the wide-format sector of flatbed, combination and roll-fed systems is certain to continue. But the overall consensus shows a strong future for LEDs as a practical alternative and, given the general shift away from Hg in all sectors of manufacture, it’s fortunate that there is curing option that is already accepted within the printing industry as a whole. 

RoHS: key facts

Adopted by the EU in February 2003, RoHS stands for the Restriction of Hazardous Substances Directive, and applies to the use of hazardous materials – including  mercury as used in mercury arc lamp curing systems. 

Closely allied to the Waste Electrical and Electronic Equipment (WEEE) Directive, RoHS applies to electrical and electronic products and all those applicable within the EU market after 1 July, 2006 must pass the necessary compliance.

RoHS 2 incorporates relatively minor changes that can affect “control and monitor instruments”, now proposed for inclusion and no longer exempt from compliance. No additions have been made to the original six substances which are lead (Pb), cadmium (Cd), hexavalent chromium (CrVI), polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) and mercury (Hg).